You're right (kind of). I still think the question was too subjective, but since I took the time to answer I should have added something informative. So here goes:

1. If you are not going to modify your engine then there is no reason to go over 60mm. This will keep the exhaust quieter (nicer?) and still allow you the maximum power gains.

2. The stock ITR catback has two resonators. Resonators make the system quieter without restricting the flow.

3. If you are removing the stock cat don't replace it with just a test pipe. This will make the exhaust MUCH louder. Instead you can get a high flow cat which won't effect the sound at all. Or you could use another resonator or small glass pack muffler; these will make the exhaust louder, but not as bad as just a test pipe.

4. The stock ITR muffler is huge and chambered. This makes it quieter, but straight through mufflers flow much better. This is a trade off that depends on what you want from your car. There are also "twin loop" type exhausts that are quieter, but do not flow as well.

If you are going to spend the money on a new exhaust, then you might as well get some upgraded performance (better flow rate). So I would say get a 60mm catback system with dual resonators and a straight-through muffler. Then get a high-flow cat, or else have the muffler shop make a test pipe from a small glasspack muffler or resonator.

And finally don't fall for the silencer trick. What's the point of getting a free flowing exhaust if you're going to drive around with it plugged up. I could only see doing that if you took your car to the track regularly. Then you could have a full out race exhaust and plug it up when you were on the streets.